This invention relates to a tin-containing ferrous composite powder useful as the raw material of a ferrous sintered alloy and a method of producing the ssame and to a tin-containing ferrous sintered magnetic material.
With the development of powder metallurgy, ferrous sintered materials are increasingly used in mechanical and electrical industries. Since various properties are required of practical ferrous sintered materials, a wide variety of alloying elements have been tested and employed to meet such requirements.
Conventional raw material powders for producing ferrous sintered alloys include mixtures of an iron powder with powders of desired alloying elements and alloy iron powders containing desired alloying elements all in prealloyed form. An advantage of a merely mixed powder is good compressibility which is exhibited at the stage of compacting the powder into a desired shape prior to sintering. This advantage is derived from softness of the unalloyed iron powder as the primary ingredient of the mixed powder. However, it is a disadvantage that the mixed powders of the alloying elements tend to segregate to result in nonuniform alloying with iron at the stage of sintering. In the case of a prealloyed iron powder there is little problem as to the uniformity of alloying. However, alloy iron powders are generally inferior in compressibility due to hardening of the powder particles by alloying.
To solve such problems of conventional ferrous powders for sintering, there are proposals for partially alloyed ferrous powders in which alloying elements are partly bonded to iron particles without significantly diffusing into iron: e.g. Japanese patent application publication No. 45-9649 (1970: bond of Mo, Cu and Ni to iron powder) and Japanese patent application provisional publication No. 53-92306 (1978: bond of Cu to iron powder). According to these proposals, a mixture of an iron powder and alloying element source powders is heated to accomplish partial bonding of particles of the latter powders to the iron particles. However, for a success in the desired bonding of the particles without diffusion of the alloying elements into the iron particles, the conditions of the heating must be determined within a very narrowly limited range and the freedom in selecting alloying elements is also restricted. Furthermore, even though the desired partial bonding of the particles is attained there is a possibility of separation of the once bonding alloying element particles from the iron particles during subsequent handling of the partially alloyed powder, and such breaking of bonds between the particles will become a cause of segregation at the stage of sintering this powder.
Meanwhile, there is a trend to use ferrous sintered materials for some parts of electrical devices such as iron cores of motors in view of soft magnetism the sintered materials possess. In general, such parts of ferrous sintered materials are produced by a powder metallurgical method including the steps of adding about 0.15-1.0 wt% of a powdery lubricating agent such as zinc stearate to a raw material powder, which may be either an iron powder or an alloy iron powder and may optionally contain secondary powders as sources of desired alloying elements, compacting the resultant powder mixture in a metal die under a pressure of about 3000-10000 kg/cm.sup.2, and sintering the compacted material in a nonoxidizing atmosphere usually at a temperature of 1000.degree.-1350.degree. C. Where necessary the sintered body is precisely shaped by sizing or machining.
An advantage of ferrous sintered materials over the conventional iron core materials typified by laminated silicon steel sheet is a large freedom in shaping. However, it is a serious disadvantage of ferrous sintered materials that they are inferior in the alternating-current magnetic characteristics due to being very large in the iron loss. The reason is because eddy current is liable to be induced in the sintered part, which is an integral member different from a laminate, so that an eddy-current loss becomes extraordinarily great.
Therefore, efforts have been made to decrease the eddy-current loss by increasing the electrical resistances of ferrous sintered materials by effective alloying. Among various alloying elements for iron, silicon has been deemed most effective for increasing the electrical resistance. Accordingly it has long been tried to produce Si-containing ferrous sintered parts of good properties by initially mixing either elemental silicon powder or ferrosilicon powder with iron powder. However, full success has not been obtained because the addition of Si causes lowering of the green density of the raw material for sintering and, therefore, results in lowering of the sintered density. Sintered parts low in the sintered density are naturally low in the magnetic flux density. As a solution to this problem, Japanese patent application publication No. 40-12045 (1965) proposes to add phosphorus together with silicon to an iron powder with a view to improving the manner of sintering of the raw material powder and consequently enhancing the sintered density and magnetic characteristics of the products. Actually, fairly high sintered densities, viz. 7 g/cm.sup.3 or above, can be attained by the addition of P when the content of Si is up to about 3 wt%. However, when the content of Si is more than 3 wt% it is impossible to attain a sufficiently high sintered density even though a considerable amount of P is added.